Project Summary The development of vaccines that can specifically generate cell mediated immunity has remained a major challenge in the field of vaccinology for both microbial pathogens and cancer. Prior work has shown that the use of replication deficient live microbes is one way to generate a potent pathogen-specific CD8+ T cell response. For example, even a single low dose of the non-replicating vaccine CPS strain of Toxoplasma gondii elicits a robust parasite specific CD8+ T cell response that provides long-lived protective immunity. However, many questions remain about how the immune system recognizes and processes the antigens from these attenuated organisms to prime and expand parasite-specific T cells. Prior work has shown that the CD8+ T cell response is dependent on the Batf3-dependent dendritic cells (DC1), which are commonly involved in antigen cross-presentation to CD8+ T cells as well as IL-12 production. Paradoxically, preliminary data indicates that DC1 are not required for early CD8+ T cell priming, but are required to generate a protective CD8+ T cell response. Importantly, the loss of the autophagy pathway in DC also results in a failure to generate protective CD8+ T cells. These studies emphasize two important questions: 1) what priming-independent role do DC1 play in generating a CD8+ T cell response, 2) what autophagy- dependent pathway in DC1 is required for the expansion and differentiation of CD8+ T cells? The autophagy pathway has been implicated as being required for leukocyte survival in inflammatory setting as well as an alternative pathway for the processing and presentation of parasite antigen. The studies proposed here will use a variety of unique microbial tools (fluorescent reporters, parasite auxotrophs, Cre- exressing parasites) and mouse strains (Batf3-/-, unique reporters, and mice which do not express MHCI H- 2Kb on DC1) combined with our imaging expertise to address I. Do DC1 interact directly with parasite- specific CD8+ T cells in a peptide-MHCI dependent fashion to provide the signals required for T cell expansion and differentiation and II. What autophagy-dependent processes in DC1 are required for the generation of protective effector CD8+ T cells.